Athletes constantly need to improve the fundamental skills of their sport. Game play is a very inefficient way of doing this, so many training aids have been developed.
For example, in baseball there are batting cages. A batting cage normally consists of a completely enclosed room with the walls being made of chain link fence, or the like. The baseball batter stands at one end of the room, and a pitching machine is positioned an appropriate distance from the batter. The machine pitches balls to the batter and the batter tries to hit the pitches. When hit, a ball strikes a chain link wall of the batting cage and falls to the ground where it can be easily retrieved and returned to the pitching machine when the exercise is over. In a matter of minutes the batter obtains far more practice batting than he would during a baseball game that lasted for hours. In addition, the player obtains this practice alone. No pitcher, catcher, fielder, nor field is required. However, the batting cage does usually require a relatively large area.
For golf, driving ranges accomplish a similar goal. A golfer can practice his swing for driving and chipping much more efficiently than he could during the course of a normal game. Here again, driving ranges require a very large open area.
Other sports have their own specialized equipment which allow one player to practice game fundamentals. However, all of these training aids do a poor job of simulating game play. The batting cage, for example, does not indicate to the batter whether a batted ball would have resulted in a hit or an out. Also, a typical batting cage cannot adjust for different strategies encountered when playing against an opposing team.